Haemophilus, genus of bacteria that, though often coccobacillus, vary widely in shape in the family Pasteurellaceae. All species of Haemophilus are strict parasites of warm-blooded animals, including humans—where they occur particularly in the mouth, upper respiratory tract, and intestinal tract; they also occur in certain cold-blooded animals. All Haemophilus are gram-negative, aerobic or facultative anaerobic, and nonmotile and require a growth factor that is found in blood. They are minute in size, H. influenzae measuring 0.3 micrometre across and up to 2 micrometres long.
Several species of Haemophilus are pathogenic (disease causing) in humans and other animals. H. gallinarum causes infectious coryza in fowl. H. parasuis causes Glasser disease in pigs. H. ducreyi causes a venereal disease in humans known as chancroid, or soft chancre. H. influenzae is a cause of various infections in humans, including ear infections, skin infections, and severe blood infections. H. influenzae type B causes Haemophilus b, or Hib, disease in children, which is characterized by fever, lethargy, and, in severe cases, pneumonia and meningitis (inflammation of the lining of the brain and spinal column).